It has previously been reported that during fermentation in normal growth media, Streptomyces mediterranei synthesizes a family of antibiotics collectively referred to as the rifamycin complex (P. Sensi, et al., Antibiotics Annual 1959-1960, page 262). Subsequent work revealed that the addition of sodium diethyl barbiturate to the culture medium resulted essentially in the formation of a single fermentation product, rifamycin B (Margalith P. and Pagani H., Applied Microbiology, 9, 325, 1961).